Late degenerative changes in simultaneously implanted and explanted tricuspid and mitral porcine (Hancock) bioprostheses have never been compared. Little information is available concerning degenerative changes in porcine bioprostheses in the tricuspid position implanted in humans. Previously, we compared morphologic alterations in simultaneously implanted and explanted porcine bioprostheses in the aortic and mitral positions. Aortic bioprostheses degenerated at a less rapid rate than the corresponding mitral bioprostheses. The present study examined the relative rates of degeneration of bioprostheses in the tricuspid and mitral positions. Six patients had pairs of valves implanted and explanted simultaneously at a mean of 3.75 yrs. The major findings were degenerative changes in all 12 valves examined with an accelerated rate for bioprostheses in the mitral position.